New technologies for the fabrication of balance springs from micromachinable materials, such as silicon, silicon oxide, DLC or similar, implemented by MEMS, DRIE, LIGA or similar methods, have made significant progress, and the springs thus fabricated have significant advantages:                geometrically, they are very close to nominal geometries, much more consistent than the steel springs of the prior art;        they have more stable mechanical properties than steel springs, due to the absence of plastic deformation;        they are non-magnetic;        they can be made on several stages (especially double springs), and/or with a terminal curve, with increased precision compared to the prior steel spring technology.        
However, with this type of spring made of micromachinable material, fine adjustment is greatly limited, because the spring does not have an independent system for adjustment of the mean frequency and isochronism, which can be used after assembly.
If the isochronous gradient is changed by a modification of the active length, the mean frequency of the corresponding sprung balance assembly is greatly modified, and correction of the balance wheel screws is then generally not sufficient to compensate for this error.
However, a conventional steel balance spring can be adjusted independently, even after assembly, as regards its mean frequency and isochronous gradient, through the use of the index assembly, and because plastic deformation of the balance spring is possible.
Moreover, fabrication systems cannot fabricate sprung balances that are sufficiently close to the nominal geometry to ensure increased chronometric performance, since an error on the order of magnitude of a ppm remains even with extremely careful machining.
FR Patent 2024511, in the name of PORTESCAP, describes a regulating device with a double balance spring, the two balance springs being arranged for mutual temperature compensation.
CH Patent 256274, in the name of ERNEST BOREL, describes an index assembly with a movable stud.
EP Patent 2104006, in the name of NIVAROX, describes a one-piece double balance spring with a common collet, notably made of silicon.
EP Patent 2233989, in the name of ULYSSE NARDIN, describes a balance spring whose outer coil is divided into two strips defining an oblong opening for housing an adjustment member other than the stud.
CH Patent 704677, in the name of MHVJ, describes a tourbillon with a particular stud arrangement, arranged to limit the lateral displacement of the balance spring in the event of a shock.
CH Patent 45160, in the name of MONTRES INVAR, describes a balance spring with two stages with independent outer coils.
EP Patent 2781967, in the name of NIVAROX, describes an escapement mechanism with a stud whose position is movable and adjustable.